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United States Patent |
5,208,456
|
Appel
,   et al.
|
May 4, 1993
|
Apparatus and system for spot position control in an optical output
device employing a variable wavelength light source
Abstract
In an optical output device wherein a beam of light is generated and
focused to a spot upon an image plane, such as a photoreceptor in a
xerographic printing apparatus, an apparatus for controlling the position
of the spot in the slow scan direction of the image plane including a
light source capable of emitting the beam of light at a selected one of at
least two selectable wavelengths and beam deflecting means for deflecting
the beam of light an amount which depends on the wavelength of the beam of
light, the amount of deflection determining the position of the spot in
the slow scan direction on the image plane. The light source may be of the
solid state laser type, and may emit a plurality of beams of light the
spots from which may be individually or together selectively positioned in
the slow scan direction on the image plane. The beam deflecting element
may be a semiconductor prism, for example of AlGaAs, and have a
controllable bias applied thereto to allow further control of spot
positioning.
Inventors:
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Appel; James J. (Brighton, NY);
Paoli; Thomas L. (Los Altos, CA)
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Assignee:
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Xerox Corporation (Stamford, CT)
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Appl. No.:
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747166 |
Filed:
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August 19, 1991 |
Current U.S. Class: |
250/236; 347/260; 358/480; 359/204; 359/217 |
Intern'l Class: |
H01J 003/14; G01D 009/42 |
Field of Search: |
250/235,236
346/108
359/204,211,212-215,217-219
358/481,480,494,496
|
References Cited
U.S. Patent Documents
4176907 | Dec., 1979 | Matsumoto et al. | 359/217.
|
4250465 | Feb., 1981 | Leib | 331/94.
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4600837 | Jul., 1986 | DiStefano et al. | 250/235.
|
4770507 | Sep., 1988 | Arimoto et al. | 359/211.
|
5006705 | Apr., 1991 | Saito et al. | 346/108.
|
5018808 | May., 1991 | Meyers et al. | 250/235.
|
5068677 | Nov., 1991 | Matsuura et al. | 359/217.
|
5089908 | Feb., 1992 | Jodoin et al. | 250/236.
|
Other References
Bestenreiner, F., U. Greis, J. Helmberger, and K. Stadler, Visibility and
Correction of Periodic Interference Structures in Line-by-Line Recorded
Images, Journal of Applied Photographic Engineering, 2:2, Spring 1976, pp.
86-92.
Urbach, John C., Tibor S. Fisli, and Gary K. Starkweather, Laser Scanning
for Electronic Printing, Proceedings of the IEEE, 70:6, Jun. 1982, pp.
597-618.
Sprague, Robert A., John C. Urbach, and Tibor S. Fisli, Advances in Laser
and E-O Printing Technology, Laser Focus/Electro-Optics, Oct. 1983, pp.
101-109.
|
Primary Examiner: Nelms; David C.
Assistant Examiner: Messinger; Michael
Attorney, Agent or Firm: Small; Jonathan A.
Claims
What is claimed is:
1. In an optical output device wherein a beam of light is generated and
focused to a spot upon an image plane means, an apparatus for controlling
the position of the spot in the slow scan plane of the image plane means,
comprising:
a light source capable of emitting the beam of light at a selected one of
at least two selectable wavelengths; and
beam deflecting means for deflecting the beam of light an amount which
depends on the wavelength of the beam of light, the amount of deflection
determining the position of the spot in the slow scan plane on the image
plane means.
2. The apparatus of claim 1, wherein the light source is a variable
wavelength laser.
3. The apparatus of claim 1, wherein the beam deflecting means is a prism.
4. The apparatus of claim 3, wherein the prism is composed of glass.
5. The apparatus of claim 3, wherein the prism is composed, at least in
part, of a crystalline material.
6. The apparatus of claim 5, wherein the crystalline material is AlGaAs.
7. The apparatus of claim 5, further including means to vary a bias applied
to the crystalline portion of the prism such that a selected change in
bias applied to the semiconducting portion of the prism results in a
selected change in the refractive index of, and hence the amount the beam
is deflected by, the prism.
8. The apparatus of claim 1, wherein the beam deflecting means is a
diffraction grating.
9. In an optical output device capable of simultaneously emitting a
plurality of light beams which are focused to individual spots upon an
image plane means, an apparatus for controlling the position of the spots
in the slow scan plane of the image plane means, comprising:
a light source capable of emitting the beams of light at a selected one of
at least two selectable wavelengths; and
beam deflecting means for deflecting the beam of light an amount which
depends on the wavelength of the beam of light, the amount of deflection
determining the position of the spot in the slow scan plane on the image
plane means.
10. The apparatus of claim 9, wherein the light source is variable
wavelength laser.
11. The apparatus of claim 10, wherein the beam deflecting means is a
prism.
12. The apparatus of claim 11, wherein the prism is composed of glass.
13. The apparatus of claim 11, wherein the prism is composed, at least in
part, of a semiconducting material.
14. The apparatus of claim 13, wherein the semiconductor material is
AlGaAs.
15. The apparatus of claim 13, further including means to vary a bias
applied to the semiconducting portion of the prism such that a selected
change in bias applied to the semiconducting portion of the prism results
in a selected change in the refractive index of, and hence the amount the
beam is deflected by, the prism.
16. The apparatus of claim 10, wherein the beam deflecting means is a
diffraction grating.
17. In an optical output device capable of simultaneously emitting a
plurality of light beams which are focused to individual spots upon an
image plane means, an apparatus for independently controlling the position
of each spot in the slow scan plane of the image plane means, comprising:
a light source capable of emitting each beam of light at an individually
selected one of at least two selectable wavelengths; and
beam deflecting means for deflecting each beam of light an amount which
depends on the wavelength of the beam of light, the amount of deflection
determining the position of the spot in the slow scan plane on the image
plane means.
18. The apparatus of claim 17, wherein the light source is a variable
wavelength laser.
19. The apparatus of claim 17, wherein the beam deflecting means is a
prism.
20. The apparatus of claim 19, wherein the prism is composed of glass.
21. The apparatus of claim 19, wherein the prism is composed, at least in
part, of a semiconducting material.
22. The apparatus of claim 21, wherein the semiconductor material is
AlGaAs.
23. The apparatus of claim 22, further including means to vary a bias
applied to the semiconducting portion of the prism such that a selected
change in bias applied to the semiconducting portion of the prism results
in a selected change in the refractive index of, and hence the amount the
beam is deflected by, the prism.
24. The apparatus of claim 18, wherein the beam deflecting means is a
diffraction grating.
25. A system for outputting an optical signal, comprising:
a light source capable of emitting a beam of light at a selected one of at
least two selectable wavelengths;
beam deflecting means located in the path of said light beam for deflecting
the beam of light an amount which depends on the wavelength of the beam of
light;
image plane means for receiving the light beam; and
means for focusing the light beam to a spot on the image plane means;
whereby, a shifting of the light beam from one to another of the at least
two selectable wavelengths causes the beam deflecting means to change the
amount that the beam is deflected and thereby reposition the spot on the
image plane means.
26. The system of claim 25, further including means for scanning the light
beam in a raster fashion in a fast scan plane, and further wherein said
beam deflecting means is oriented such that a shifting of the light beam
from one to another of the at least two selectable wavelengths causes the
beam deflecting means to change the amount that the beam is deflected and
thereby reposition the spot on the image plane means in a slow scan
direction which is orthogonal to the fast scan direction.
27. The system is claim 25, wherein the light source comprises a variable
wavelength laser, and further wherein the system includes means for
selecting one of at least two selectable wavelengths of the output of the
laser.
28. The system of claim 27, wherein the the beam deflecting means is a
prism.
29. The system of claim 28, wherein the prism is composed of glass.
30. The system of claim 28, wherein the prism is composed, at least in
part, of a semiconducting material.
31. The system of claim 30, wherein the semiconductor material is AlGaAs.
32. The apparatus of claim 30, further including means to vary a bias
applied to the semiconducting portion of the prism such that a selected
change in bias applied to the semiconducting portion of the prism results
in a selected change in the refractive index of, and hence the amount the
beam is deflected by, the prism.
33. The system of claim 27, wherein the beam deflecting means is a
diffraction grating.
34. The system of claim 27, further including means for determining the
existence and extent of any error in the position of the spot in the slow
scan plane, and means for application of such determination to the means
for controlling the wavelength of the output of the laser, such that an
appropriate shift of the light beam from one to another of the at least
two selectable wavelengths is caused, and further that such a shift in
wavelength causes the beam deflecting means to change the amount that the
beam is deflected and thereby reposition the spot on the image plane means
in the slow scan direction to compensate for the determined existence and
extent of error.
35. The system of claim 34, further including means for determining the
need for, and the extent of, the application of predetermined spot
position correction, and means for application of such determination to
the means for controlling the wavelength of the output of the laser, such
that an appropriate shift of the light beam from one to another of the at
least two selectable wavelengths is caused, and further that such a shift
in wavelength causes the beam deflecting means to change the amount that
the beam is deflected and thereby reposition the spot on the image plane
means in the slow scan direction in accordance with the predetermined
correction.
36. The system of claim 27, further including means located in the path of
said light beam for modulating the light beam in accordance with a data
signal applied to said means for modulating.
37. A system for outputting an optical signal, comprising:
a variable wavelength laser light source capable of emitting a beam of
laser light at a selected one of at least two selectable wavelengths;
means for selecting the one of at least two selectable wavelengths of the
output of the laser light source;
rotating output scanning means located in the path of said light beam for
scanning the light beam in a raster fashion in a fast scan plane;
a beam deflecting element located in the path of said light beam whose
index of refraction varies as a function of the wavelength of light
passing therethrough, for deflecting the beam of light in a slow scan
plane, which is orthogonal to the fast scan plane, an amount which depends
on the wavelength of the beam of light;
image plane means for receiving the light beam; means for focusing the
light beam to a spot on the image plane means;
means for determining the existence and extent of any error in the position
of the spot on the image plane means;
means for determining the need for, and the extent of, the application of
predetermined spot position correction; and
means for application of such determinations of the existence and extent of
any error and the need for, and extent of, the application of
predetermined spot position correction to the means for selecting the
wavelength of the output of the laser such that an appropriate shift of
the light beam from one to another of the at least two selectable
wavelengths is caused, and further that such a shift in wavelength causes
the beam deflecting element to change the amount that the beam is
deflected and thereby reposition the spot on the image plane means in the
slow scan direction to compensate for the determined existence and extent
of error and the determined needed application of predetermined spot
position correction.
38. An improved printing apparatus, the improvement comprising:
a variable wavelength laser light source capable of emitting a beam of
laser light at a selected one of at least two selectable wavelengths;
means for selecting the one of at least two selectable wavelengths of the
output of the laser light source;
rotating output scanning means located in the path of said light beam for
scanning the light beam in a raster fashion in a fast scan plane;
a beam deflecting element located in the path of said light beam whose
index of refraction varies as a function of the wavelength of light
passing therethrough, for deflecting the beam of light in a slow scan
plane, which is orthogonal to the fast scan plane, an amount which depends
on the wavelength of the beam of light;
a photoreceptive element for receiving the light beam;
means for focusing the light beam to a spot on the photoreceptive element;
means for determining the existence and extent of any error in the position
of the spot on the photoreceptive element;
means for determining the need for, and extent of, the application of
predetermined spot position correction; and
means for application of such determinations of the existence and extent of
any error and the need for, and extent of, the application of
predetermined spot position correction to the means for selecting the
wavelength of the output of the laser such that an appropriate shift of
the light beam from one to another of the at least two selectable
wavelengths is caused, and further that such a shift in wavelength causes
the beam deflecting element to change the amount that the beam is
deflected and thereby reposition the spot on the photoreceptive element in
the slow scan direction to compensate for the determined existence and
extent of error and the determined needed and extent of the application of
predetermined spot position correction.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of optical output
devices, and more specifically to a device providing position or
registration control of a laser spot or spots, which is achieved by
varying the output wavelength of the device's light source.
The present application relates to the following concurrently filed U.S.
patent applications: Ser. No. 07/747,889; Ser. No. 07/747,039; and Ser.
No. 07/747,176. Each of these applications is hereby incorporated by
reference.
Although applicable to a wide variety of optical output devices, the
present invention finds particular utility in Raster Output Scanning (ROS)
apparatus. Therefore, the following details and descriptions begin with a
background of the present invention in terms of ROS apparatus. ROS has
become the predominant method for imparting modulated light information
onto the photoreceptor in printing apparatus used, for example, in digital
printing, and has found some application in other image forming operations
such as writing to a display, to photographic film, etc. Consider, for
illustration purposes, what is perhaps the most common application of ROS,
digital printing. As is known, the scanning aspect thereof is
conventionally carried out by a moving reflective surface, which is
typically a multifaceted polygon with one or more facets being mirrors.
The polygon is rotated about an axis while an intensity-modulated light
beam, typically laser light, is brought to bear on the rotating polygon at
a predetermined angle. The light beam is reflected by a facet and
thereafter focussed to a "spot" on a photosensitive recording medium. The
rotation of the polygon causes the spot to scan linearly across the
photosensitive medium in a fast scan (i.e., line scan) direction.
Meanwhile, the photosensitive medium is advanced relatively more slowly
than the rate of the fast scan in a slow scan direction which is
orthogonal to the fast scan direction. In this way, the beam scans the
recording medium in a raster scanning pattern. (Although, for the purposes
of example, this discussion is in terms of ROS apparatus, as will become
apparent from the following discussion, there exists many other scanning
and non-scanning system embodiments of the present invention. However, as
a convention, the word "scan" when referring to fast and slow scan
directions will be used with the understanding that actual scanning of the
spot is not absolutely required.) The light beam is intensity-modulated in
accordance with a serial data stream at a rate such that individual
picture elements ("pixels") of the image represented by the data stream
are exposed on the photosensitive medium to form a latent image, which is
then transferred to an appropriate image receiving medium such as sheet
paper.
Data in each of the fast and slow scan directions is generally sampled. The
sampling rate of the slow scan direction data equates to 300 lines per
inch or more in many printing apparatus. It has been shown that errors in
the slow scan direction of as small as 1% of the nominal line spacing may
be perceived in a half tone or continuous tone image. This implies a need
for a high degree of spot position control in the slow scan direction on
the image plane, especially in such applications as multiple beam and
multiple ROS color printers where control of the position of multiple
spots is critical. Furthermore, high resolution printing, on the order of
600 spots per inch or higher demands very accurate spot positioning.
Errors in the spot position in the slow scan direction arise from many
sources, including polygon and/or photosensitive medium motion flaws,
facet and/or image plane (e.g., photosensitive medium) surface defects,
etc. These errors are most commonly addressed by passive or active in-line
optics. Positional errors which extend over an entire scan line are most
commonly compensated for by retarding or advancing the start of scan by
one or more scan lines (this correction being limited to errors of whole
multiples of a scan line spacing). See, for example, Advances in Laser and
E-O Printing Technology, Sprague et al., Laser Focus/Electro-Optics, pp.
101-109, October 1983. Another approach employing passive optics is the
use of extremely high quality optical and mechanical elements. This
necessarily implies higher overall costs, and possible limitations on the
durability of the system. Still another example of passive optical
correction is the system disclosed in U.S. Pat. No. 4,040,096, issued Aug.
2, 1977 to Starkweather which accommodates a basic polygon ROS structure
having runout and/or facet errors (both scanning errors in the slow scan
direction) by locating a first cylindrical lens in the pre-polygon optical
path, which focuses the beam in the slow scan direction onto the facet,
and a second cylindrical lens in the post-polygon path, which focuses the
facet onto the desired image plane. Toroidal elements and concave mirrors
have also been used to accomplish the same function.
Active compensation for process scan direction errors usually involves a
closed loop and/or memory-fed compensation system. A closed loop
acousto-optical (A-O) compensation system is discussed in Laser Scanning
for Electronic Printing, Urbach et al., Proceedings of the IEEE, vol. 70,
no. 6 June 1982, page 612, and the reference cited therein. As discussed
in this reference, a slow scan spot position detector is placed in the
scan line which, together with related processing apparatus, is capable of
quantifying the slow scan displacement. An A-O element is disposed in the
optical path whose refractive index may be varied by establishing therein
an acoustic wave. A variation in the acoustic wave generated in the A-O
element is accompanied by a variation in the dispersion angle (that is,
the angle of the output beam relative to the angle of the input beam). The
slow scan displacement information from the detector and processing
apparatus is fed to the acoustic wave generating portion of the A-O
device, which may then control the slow scan direction position of the
scan line in response to the displacement information. Further, the
control information for certain recurrent displacement errors may be
measured in advance and synchronized with the angular motion of the
rotating polygon, as discussed in the above reference. See also
Bestenreiner et al., Visibility and Correction of Periodic Interference
Structures In Line-by-Line Recorded Images, J. Appl. Phot. Eng., vol. 2,
no. 2, pp. 86-92, Spring 1976.
One technology which, although it is directed to a method of scanning, as
opposed to addressing slow scan direction errors, is nonetheless relevant
is disclosed in Fast Dispersive Beam Deflectors and Modulators, Filinski
and Skettrup, IEEE Journal of Quantum Electronics, vol. QE-18, no. 7, pp.
1059-1062, July 1982. As briefly described therein, a static optical
element having dispersion characteristics which vary as a function of the
wavelength of the incident light can be utilized to scan in one dimension
by varying the output wavelength of the light source. Various types of
static dispersive elements are mentioned therein including prisms and
gratings, although no details about incorporation of this type of scanning
element into a complete scanning system are provided. Nor is there any
mention in that reference about employing the described apparatus to
control slow scan direction spot position.
There is presently a need in the art for spot position control apparatus
and methods which provide improved continuous, very high resolution
deflection of an optical beam in the slow scan direction.
Shortcomings of spot position control schemes known in the art include the
complexity, cost and/or the difficulty of manufacture of such systems. For
example, the use of high quality optics requires not only high quality
optical elements, but utmost control in the positioning of those optics,
in order to obtain the requisite very precise mechanical control
sufficient to adjust spot position 0.02 mm or less, required in many
cases. In order to achieve this level of spot position control with the
aforementioned acousto-optic modulators, an acoustic wave must be
established and maintained with great precision. The acousto-optic
modulators employed are relatively quite expensive, and require an
associated accurate high frequency signal generator and related
electronics to produce and maintain the acoustic waves.
Two further disadvantages of many prior art spot position control schemes
are the speed and precision at which they are capable of operating. For
example, three of the most common ROS schemes, cylinder lenses, rotating
mirrors, and translating roof mirrors are generally too slow to correct
for motion quality errors or line-to-line errors, while rotating mirrors
and translating roof mirrors are also large and therefore difficult to
move precisely and quickly.
SUMMARY OF THE INVENTION
The present invention provides a novel apparatus for controlling the spot
position or registration in the slow scan direction in an optical output
system which overcomes a number of problems and shortcomings of the prior
art. Spot position refers to the location that a light beam is incident
upon an image plane, and spot registration refers to the location that the
light beam is incident on that image plane relative to other spot
positions (for example in overwriting a spot for tone, position, color, or
control of other parameters). However, for simplicity of explanation, any
reference to control of spot position will include control of spot
registration, unless otherwise noted. In general, the spot position
control is provided by interposing in the image path a beam deflecting
element which deflects the beam an amount which varies as a function of
the wavelength of the light it deflects. A variable wavelength light
source, such as a multiple wavelength solid state laser, may then be
employed as the light source so as to allow controlling of the spot
position. Spot position control may be achieved for either a single spot
or for multiple spots where the position of each spot relative to the
other spots is maintained. Depending on the output parameters of the
optical output apparatus embodying the present invention, spot position
control may be achieved on a pixel-by-pixel basis.
One embodiment of the present invention is a ROS apparatus which includes,
inter alia, a variable wavelength light source, typically a multiple
wavelength solid state laser, for emitting a light beam at a selected
wavelength, means for controlling the wavelength of output of the light
source, optical beam deflecting means, such as a prism of glass or
semiconductor material, for deflecting the light beam an amount that is
determined by the wavelength of the light passing through it, means for
modulating the light beam in accordance with a data signal, means for
scanning the light beam in a raster fashion, and image plane means, such
as a photoreceptive element, for receiving the scanned light beam. Means
for determining the existence and extent of spot position errors and/or
the need for application of predetermined spot position correction may
also be included.
In operation, a light beam is generated by the light source which is
modulated in response to an image data signal. The light beam is scanned
across at least a portion of a surface of the image plane means in a fast
scan direction, as well as scanned across at least a portion of a surface
of the image plane means in a slow scan direction which is normal to the
fast scan direction. The existence and extent of error, if any, in the
position of the light beam in the slow scan direction is determined for a
part or all of the scan in the fast scan direction. Correction for any
slow scan direction error is performed by varying the output wavelength of
the light source in response to the determination of the existence and
extent of such error. This change in wavelength results in a change in the
extent which the light beam is deflected in the slow scan direction by the
deflecting means, which ultimately changes the position that the beam
strikes the image plane means.
The control of spot position on the image plane provided by the present
invention may be employed to correct for inter-line slow scan direction
positional errors by varying the output wavelength of the light source in
response to the output of a means for detecting and quantifying such
positional errors and/or in response to predetermined correction
information output from a processor controlled memory unit or the like.
Furthermore, the maximum amount of slow scan direction spot position
correction that will be required will be equal to one half of a scan line
height. Any greater amount of correction may be realized through a
combination of the above spot position control and retardation or
advancement of one or more scan lines.
The scope of the present invention and the manner in which it addresses the
problems associated with prior art methods and apparatus will become more
readily apparent from the following detailed description when taken in
conjunction with the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a side or elevation view of the general optical configuration
of an apparatus according to one embodiment of the present invention,
showing an optical beam deflecting element in the form of a prism disposed
between the light source and the rotating polygon scanning device of a
typical ROS system.
FIG. 1A shows a photoreceptive drum at the image plane of the apparatus of
FIG. 1 as might typically be employed in a xerographic printing
application of the present invention.
FIG. 2 shows a top or plan view of the general optical configuration of the
apparatus of FIG. 1, showing an optical beam deflecting element disposed
between the light source and the rotating polygon scanning device of a
typical ROS system.
FIG. 3 shows a side or elevation view of the general optical configuration
of another embodiment of the present invention, showing an optical beam
deflecting element disposed between a multiple light source and the
rotating polygon scanning device of a typical multiple beam ROS system
FIG. 4 shows a schematic representation of a ROS system for the purposes of
describing the nature and extent of the control of spot position provided
in the slow scan direction by the present invention.
FIG. 4a shows the dispersive element of one embodiment of the present
invention labeled for calculation of the angle .alpha. of the element.
FIG. 5 shows in detail an optical beam deflecting element which may be
employed in the present invention to allow controllable spot registration
in a ROS system.
FIG. 6 shows a side or elevation view of the general optical configuration
of another embodiment of the present invention, showing an optical beam
deflecting element in the form of a semiconductor prism having a thin
optical waveguide disposed between the light source and the rotating
polygon scanning device of a typical ROS system.
FIG. 7 shows a top or plan view of the general optical configuration of the
apparatus of FIG. 6, showing an optical beam deflecting element disposed
between the light source and the rotating polygon scanning device of a
typical ROS system.
FIG. 8 shows a side or elevation view of the general optical configuration
of another embodiment of the present invention, showing an optical beam
deflecting element in the form of a diffraction grating disposed between
the light source and the rotating polygon scanning device.
FIG. 9 shows one embodiment of a solid state laser capable of emitting at
one of a number of selectable wavelengths.
FIG. 10 shows another embodiment of a solid state laser capable of emitting
at one of a number of selectable wavelengths.
FIG. 11 shows a side or elevation view of the general optical configuration
of another embodiment of the present invention, showing an optical beam
deflecting element in the form of a prism disposed between the light
source and a non-scanning modulation device.
FIG. 12 shows a side or elevation view of the general optical configuration
of an apparatus according to the first embodiment of the present
invention, further including means for detecting errors in the position of
the photoreceptive drum and for feeding a measure of the error back to the
laser light source as a control signal for adjusting the position of the
laser beam emerging from the optical beam deflecting element.
FIG. 13 is a flow diagram of one embodiment of the present invention for
determining and correcting for slow scan direction errors on the fly, and
for compensating for predetermined slow scan direction spot position
errors.
In general, like reference numerals will be used to denote like elements as
between each of the aforementioned figures.
DETAILED DESCRIPTION
A detailed description of a first embodiment of the present invention is
presented herein with reference to FIGS. 1 and 2, which shows,
respectively, slow scan plane and fast scan plane views of a scanning
apparatus 10. Apparatus 10 is a raster output scanning device of the type
which may, for example, output a scanned modulated optical signal to a
photoreceptive drum 12, such as that shown in FIG. 1A, for use in a
xerographic printing process. Alternatively, apparatus 10 may output a
scanned modulated optical signal to a display device, a photographic
device or other application employing such a scanned modulated optical
signal.
Apparatus 10 includes a light source 14, such as a solid state laser or
array of lasers, which produces a diverging beam of coherent light 16. In
the path of beam 16 are a spherical lens 18, a cylindrical lens 20, which
has power only in the slow scan plane, optical beam deflecting element 22,
which is described in further detail below, scanning device 24, which is
shown as a rotating polygon having at least one reflective facet 26 (but
which may also be rotating hologram, rotating diffraction grating, etc.),
first spherical lens 28, and toroidal lens 30. The path of beam 16
terminates at image plane 32, which may be a line on the aforementioned
rotating photoreceptive drum 12 (FIG. 1A), a surface of a ground glass or
other type of display screen, a photosensitive film, etc.
Spherical lens 18 serves to collimate the diverging beam 16. Cylindrical
lens 20 serves to focus beam 16 in the slow scan plane onto facet 26 of
scanning device 24. Since beam 16 is not focussed in the fast scan plane,
it is appears on facet 26 as a line which extends across the entire width
of facet 26.
Prior to its incidence on facet 26, beam 16 passes through, and is
deflected by optical beam deflecting element 22. The role of element 22 is
to provide beam deflection by an angle which is determined by the
wavelength of the incident light. The structure and details of optical
beam deflecting element are discussed further below. Beam 16, having been
appropriately deflected is then reflected by facet 26 so as to pass
through spherical lens 28. Since the beam converges on facet 26, upon
reflection it diverges. Therefore, lenses 28 and 30 are employed to
refocus the beam to a circular or elliptical cross-section onto image
plane 32, and to correct for scan nonlinearity (f-theta correction).
Toroidal lens 30, or an equivalent thereto (such as a cylindrical mirror)
further corrects for wobble (scanner motion or facet errors).
Thus, if polygon 24 rotates in a clockwise fashion, as shown in FIG. 2, a
beam reflected from one of its moving facets will be caused to scan across
the image plane 32, as indicated by the arrow. By modulating the beam, for
example by modulating the current applied to the laser itself from below
to above the lasing threshold, as known in the art, a scanned modulated
single beam of general application results. If the image plane 32
comprises the line on the rotating photoreceptive drum 12 of FIG. 1A, and
the rotation of drum 12 and the modulation and scanning of the beam are
properly coordinated, a ROS printer device may be realized.
An embodiment of the present invention allowing simultaneous and
independent spot position or registration control for a plurality of spots
is shown in FIG. 3. Apparatus 52 includes multiple light sources 54a, 54b,
such as independent solid state lasers, or a monolithic multiple beam
solid state laser, which produce diverging beams of coherent light. The
wavelenghs of the beams need not to be the same, since their positions
relative to one another are independently controlled in order to position
each spot at the start of scan. As viewed in the slow scan plane of FIG.
3, beams 56a and 56b pass through spherical lens 58 which collimates the
beams in the fast scan plane. The collimated beams then pass through a
cylindrical lens 66, and an optical beam deflecting element 62 so that
they illuminate a facet 66 of scanning device 64. Cylindrical lens 60,
which has power only in the slow scan direction, focuses the beams 56a and
56b through element 62 onto facet 66, each beam being focused only in the
slow scan direction so that each beam focuses on facet 66 as a line. Facet
66 reflects each of the beams, which at this point diverge, to lens 68.
Lenses 68 and 70 refocus the beams onto image plane 72 and correct scan
nonlinearity. Lens 70 is a toroidal wobble correcting element. As before,
the image plane may be ground glass, a viewing screen, a photosensitive
material (film, electrostatic photoreceptor, etc.), or other image plane
viewing or receiving medium. Modulation of the beams may be conveniently
achieved by directly modulating the output of each light source, for
example by modulating the current applied to the laser itself from below
to above the lasing threshold, as known in the art.
Due to the similarity between the structure and operation of the embodiment
of FIGS. 1 and 2 and the embodiment of FIG. 3, the following description
of these embodiments shall be with regard to a single beam embodiment
(that shown in FIGS. 1 and 2) for clarity and simplicity. The following
description is, however, equally applicable to multiple beam apparatus, as
will be appreciated by those skilled in the art. Furthermore, many of the
details of the lenses and other optical and mechanical components of a
complete ROS system may be omitted for clarity since they are well known
in the art.
For the purposes of the following explanation, it will be assumed that
optical beam deflecting element 22 takes the form of an isosceles
triangular prism, as shown in FIGS. 1 and 2. Furthermore, the material
from which optical beam deflecting element 22 is formed will be assumed to
be optically transparent glass. However, it will be appreciated that
optical beam deflecting element 22 may take other forms such as a
diffraction grating, thin film or similar element where appropriate. It
will be further understood that optical beam deflecting element 22 may be
fabricated of glass, a semiconductor material such as AlGaAs, lithium
niobate, a liquid crystal, etc., as discussed in further detail below.
Also, it will be appreciated that optimal results are achieved when
element 22, in the case that it takes the form of a prism, is fully
illuminated (i.e., fully filled with light). This is because the resolving
power of the prism is inversely proportional to the width of the optical
beam, which sets a lower limit on the height of the prism. Hence, fully
illuminating the prism achieves maximum resolution. Although beam 16 is
focused on facet 26, and not on beam deflecting element 22, by lens 20,
element 22 is effectively fully illuminated by locating element 22 much
closer to lens 20 than facet 26. That is, d.sub.1 is much smaller than
d.sub.2.
FIGS. 4 and 4a show a schematic representation of apparatus 10 for the
purposes of describing the nature and extent of the control of spot
position provided in the slow scan direction by the present invention. For
the purposes of simplicity of explanation, only elements necessary to the
explanation have been shown therein.
Facilitating the control of the registration of the spot is variable
wavelength light source 14 and optical beam deflecting element 22. The
basis of this control is the feature of element 22 that its refractive
index differs for different wavelengths of the light passing through it
(or similarly, when element 22 operates by reflection, the angle of
reflection varies as a function of the wavelength of light).
Rays of optical beam 16, as illustrated by a representative ray 80, will
undergo an angular deviation .epsilon. upon refraction by element 22. The
extent of that refraction will vary as a function of the wavelength of the
light. M. Born and E. Wolf show in Principles of Optics (fifth ed.,
Pergamon Press, p. 407) that the resolving power (resolution limit) A of a
prism is given by
A=.lambda./.DELTA..lambda.=b(dn/d.lambda.) (1)
where b is the base of the prism, .lambda. is the wavelength of incident
light, and n is the index of refraction. From I. Filinski and T. Skettrup,
Fast Dispersive Beam Deflectors and Modulators, IEEE Journal of Quantum
Electronics, vol. QE-18, no. 7, July 1982), the number of resolvable
points N per unit wavelength is
dN/d.lambda.=A/.lambda. (2)
Combining the above equations (1) and (2) yields
dN/d.lambda.=b(dn/d.lambda.)/.lambda. (3)
and solving for b in terms of .lambda. we get
b=.lambda.(dN/d.lambda.)/(dn/d.lambda.) (4)
For the system shown in FIG. 4, the largest required displacement of the
image of the spot would be .+-.0.5 line or spot widths (any greater
correction may be accomplished by advancing or retarding the slow scan
motion by one or more scan lines in combination, if necessary, with
displacement provided by the present invention). Thus, the desired number
of resolvable spots is two. Assume, for the moment, that wavelengths of
770 nm and 790 nm are chosen as the low and high operating wavelengths of
the light source. Then, dN/d.lambda.=2/20=0.1 nm.sup.-1. Since the
wavelength difference of .+-.10 nm is very small compared to the operating
wavelengths, 780 nm, which is midway between the low and high wavelengths,
is chosen for the purposes of calculating an example of prism size. Assume
also that Schott SF 10 is chosen as the prism glass. For SF 10
dn/d.lambda.=9.5.times.10.sup.-5 /nm. Thus,
b=780(0.1)/(9.5.times.10.sup.-5)=8.2.times.10.sup.5 nm=0.82 mm
This is quite reasonable for a system such as that shown in FIG. 4.
Furthermore, for the system shown in FIG. 4, the diameter of the beam can
be found from
d=2w.sub.ff =2z.lambda./.pi.W.sub.0 (5)
where W.sub.ff is the 1/e.sup.2 spot radius in the far field, W.sub.O is
the beam waist, which will be assumed to be 0.002 mm for the purposes of
the present explanation, and z is the distance from the waist, which will
be assumed to be 4.02 mm for a 20X lens shown. Therefore, W.sub.ff
=4.02(780.times.10.sup.-6)/.pi.(0.002)=0.5 mm, so that d=2W.sub.ff =2(0.5
mm)=1 mm. Taking into account refraction at the prism's input, the prism
angle .alpha. may be determined
sin.sup.2 (.alpha./2)=b.sup.2 /(4d.sup.2 +n.sup.2 b.sup.2) (6)
from which a prism angle .alpha. may be found as .alpha.=40.9 degrees.
An alternative to the glass prism discussed above is a semiconductor prism
of a material such as AlGaAs. Two particular embodiments employing an
AlGaAs optical beam deflecting element are considered. The first is a
simple substitution of AlGaAs for glass. At present there are practical
limitations to such an embodiment based on the cost, internal
imperfections, etc. of mm thick platelets of this material. To the extent
that a homogeneous semiconductor prism is practicable, its operation and
design will differ little from the prism described above of glass except
that its dispersive power, i.e. dn/d.lambda., is more than 20 times
greater than glass. Hence the base of the prism and/or the wavelength
shift of the laser source can be less than required with a glass prism, as
illustrated below. However, one alternative to mm thick platelets of
semiconducting material is the epitaxial formation of a prism of
semiconducting material.
The second particular embodiment is such an epitaxially grown semiconductor
prism having a thin waveguide core as the optical element. Referring now
to FIG. 5, the optical beam deflecting element 100 of this embodiment
includes a substrate 102 having deposited thereon an n-Al.sub.y
Ga.sub.(1-y) As cladding layer 104, where y might typically be equal to
0.40, for example by MOCVD methods well known in the art. A waveguide core
106 of Al.sub.x Ga.sub.(1-x) As, where y>x and x might be 0.2, is next
deposited on layer 104 (although the waveguide core might also be a single
or multiple quantum well structure, where appropriate). Waveguide core 106
is chosen to have a wide bandgap so that it is transparent at the
wavelengths of operation. A p-Al.sub.z Ga.sub.(1-z) As cladding layer 108,
where z is typically equal to y, although it need not necessarily be so,
is then deposited on core 106. The structure is then etched down to
substrate 102 by methods known in the art to form etched facet 110. Then,
three sides of the structure are cleaved to form cleaved facets 112, 114,
and 116. Antireflective (AR) coatings 118 and 119 are next applied to
facets 110 and 116, respectively. A prism is thereby formed which serves
to selectively refract light incident thereupon as a function of the
light's wavelength. The path of the diffracted light is shown by way of
the exemplary double arrow in FIG. 3 labeled L.
For a waveguide core of Al.sub.0.2 Ga.sub.0.8 As, dn/d.lambda. at 780 nm is
calculated to be 2.24 .times.10.sup.-3 nm.sup.-1 from Casey et al.,
Refractive Index of Al.sub.x Ga.sub.1-x As, Applied Physics Letters, vol.
24, no. 2, p. 63 (1974). To facilitate design of a laser source which may
be employed in the present embodiment, a wavelength separation of 1 nm for
the two resolvable points will be assumed. This gives dN/d.lambda.=2
nm.sup.-1. From equation (4) above, the prism base b can be found as
b=780.times.2/2.24.times.10.sup.-3 =0.696 mm. From equation (6), with
n=3.58 for Al.sub.0.2 Ga.sub.0.8 As, a 1 mm beam diameter, and b=0.696
mm, the prism angle .alpha. is found to be .alpha.=25.1 degrees. To
illustrate the possibility of tradeoffs between wavelength separation and
prism size, consider an alternative design. A wavelength separation of 4
nm for the two resolvable points may be employed in the present
embodiment. This gives dN/d.alpha.=0.5 nm.sup.-1. The prism base b can be
found as b=780.times.0.5/2.24.times.10.sup.-3 =0.174 mm and the prism
angle .alpha. is found to be 9.5 degrees for Al.sub.0.2 Ga.sub.0.8 As and
a 1 mm beam diameter. The selection of other combinations for the
wavelength separation and prism base is also possible as will be
appreciated by one skilled in the art.
Since the wavelength change required for a given number of resolved spots
depends only on n and b, we fix b and tradeoff beam width for prism angle.
Table 1 indicates possible values for an Al.sub.0.2 Ga.sub.0.8 As
waveguide core, with n=3.58, and prism base b=0.696 mm.
TABLE 1
______________________________________
angle of
Beam width d
prism angle .alpha.
incidence .phi.
prism height
(mm) (degrees) (degrees) (mm)
______________________________________
0 32.4 87.2 1.19
0.5 30.0 67.9 1.30
1.0 25.2 51.3 1.56
2.0 17.0 31.9 2.33
3.0 12.3 22.6 3.23
4.0 9.53 17.3 4.17
______________________________________
Waveguide core 106 is of the type that allows propagation of a lightwave
therethrough while confining it in at least one dimension to the order of
one wavelength. The lightwave propagates, without diverging, in the
longitudinal direction of the guide since it is confined to the waveguide
core by the lower refractive index of the cladding layers. However, in
order to take advantage of such an arrangement, beam 16 must be focused
onto the input facet of the prism. An embodiment of a scanning apparatus
configured for application of the semiconducting optical beam deflecting
element 100 is shown in FIG. 6. This embodiment is substantially as
described above, with the exception that beam 16 is focused in the fast
scan plane by cylindrical lens 120, which replaces cylindrical lens 18 of
FIGS. 1 and 2. Furthermore, since element 100 behaves as a one dimensional
optical waveguide, the light focussed on its entrance aperture propagates
through it, and diverges as it leaves its exit aperture. Therefore,
cylindrical lens 122 is employed to collimate the light in the fast scan
plane prior to its arrival at facet 26.
Yet another alternative to using the aforementioned prism is the use of a
diffraction grating. Control of spot position may be achieved for smaller
wavelength shifts when using a diffraction grating due to the grating's
greater dispersive power. An example of such an embodiment is shown in
FIG. 8, where diffraction grating 150 is of the type which diffracts light
at an angle which is dependent on the wavelength of the light (diffracted
beam 152 and diffracted beam 154 strike image plane 32 at different
locations, which depend upon the wavelength of each beam).
The basis for spot control according to this invention is wavelength
control of the light emitted by light source 14. A suitable light source
for achieving spot position control is a laser source, such as a solid
state, or diode laser. The output wavelength of a diode laser can be tuned
over a range suitable for use with a glass prism, e.g., from 770 nm to 790
nm, by controlling the loss in the laser cavity. One method for
accomplishing this may be to employ a split contact laser 200 of FIG. 9,
having a small but separately contacted modulator region along the axis of
the laser. Such a laser 200 includes an amplifier region 202 and a
modulator region 204 optically interconnected by a low-loss waveguide
region 206 which together form an optical cavity. A simple method of
electrically isolating amplifier region 202 and modulator region 204 is
the low-loss waveguide region 206, although electrical isolation may also
be achieved by appropriate diffusions and/or proton bombardment as known
in the art. Bounding the optical cavity of laser 200 are facets 208 and
210, such that light from the laser emits primarily from one or the other
facet. Laser 200 will be assumed to be an individual laser for the present
illustration, but may be one element of a laser array, the array
embodiment being described elsewhere herein.
Variation of the bias level on modulator region 204 controls the amount of
loss present in the laser cavity from a maximum value L.sub.max to a
minimum value L.sub.min. Maximum loss is obtained with modulator region
204 unbiased (or reverse biased) and results from the intrinsic absorption
of the unpumped active layer (not shown). Minimum loss is obtained with
modulator region 204 forward biased, although the loss may also be
negative, i.e., the modulator region 204 providing gain. With the
modulator region 204 set for minimum loss, laser 200 is designed to emit
at the longest wavelength required. As the modulator current is decreased,
loss in the modulator region 204 is increased and the wavelength shortens.
The length of modulator region 204 is determined by the amount of
wavelength shift required. Tuning is discontinuous as the wavelength hops
from one longitudinal mode to the next. In addition, the threshold of
laser 200 is increased so that current to amplifier region 202 is
increased in order to maintain constant power output at the lasing
wavelengths. Thus, wavelengths switching at constant power is accomplished
by simultaneously switching drive currents to both the amplifier region
202 and the modulator region 204. If the wavelength shift required by the
system is small (.about.1 nm), the shortest wavelength may be obtained
with modulator region 204 forward biased. A large wavelength shift
(.about.20 nm) may require near zero current or a reverse bias applied to
modulator region 204.
The output wavelength of a diode laser can also be tuned over a range
suitable for use with a semiconductor prism, e.g., from 780 nm to 781 nm,
by controlling the index of refraction in the laser cavity. This approach
inherently produces a smaller tuning range than the loss modulator
described above, but is advantageous in that tuning is continuous (as
opposed to step-wise) and variations in the power output between the
different wavelengths are reduced. A laser 220 capable of achieving index
control is shown in FIG. 10, and includes an amplifier region 222 and a
separately contacted high bandgap modulator region 224 along the axis of
the laser between rear facet 226 and light emitting facet 228 (although
emission may be at either one of the two facets 226 or 228). In index
modulator region 224, the optical waveguide has a bandgap greater than the
bandgap of the amplifier region 222, and so does not absorb the lasing
emission. Such a low-loss region can be produced by layer disordering
techniques or by laser patterned desorption as known in the art (for
example, see U.S. Pat. Ser. No. 4,962,057, dated Oct. 9, 1990, to Epler et
al.) The index of refraction of the optical waveguide under forward bias
is a function of the injected carrier density pursuant to the free carrier
injection effect. For a detailed discussion of the free carrier injection
effect see copending U.S. patent application Ser. No. 07/747,039. For an
integrated device of this kind, we find that the index change .DELTA.n
required to shift the wavelength is given by
.DELTA.n=[n.sub.g1 L.sub.1 /L.sub.2 +n.sub.g2 ].DELTA..lambda./.lambda.(7)
where L.sub.1 and L.sub.2 are the lengths of the amplifier and modulator
regions, respectively, and n.sub.gi .ident.n.sub.i -.lambda.dn.sub.i
/d.lambda. is known as the group refractive index in the amplifier region
(i=1) or the modulator region (i=2) and .lambda. is the mean wavelength of
operation. For AlGaAs, laser measurements give n.sub.g1 =4.24 at 780 nm.
From the experimental data of Casey et al., Applied Physics Letters vol.
24, no. 63 (1974), we calculate n.sub.g2 =1.83 for Al.sub.0.2 Ga.sub.0.8
As at 780 nm. For .DELTA..lambda.=1 nm, L.sub.1 =250 .mu.m and L.sub.2
=1000 .mu.m,
.DELTA.n.sub.2 =[4.24.times.(250/1000)+1.83].times.[1.28.times.10.sup.-3
]=3.7.times.10.sup.-3
From Casey and Parish, Heterostructure Lasers Part A: Fundamental
Principles, p. 31 (Academic Press 1978), the index change at 780 nm from
the free carrier plasma in .DELTA.n=1.23.times.10.sup.-21 /cm.sup.3
.times..DELTA.N, where .DELTA.N is the change is carrier density. Thus, a
carrier density change of
.vertline..DELTA.N.vertline.=3.29.times.10.sup.18 /cm.sup.3 will produce a
.DELTA.n.sub.2 sufficient to shift the wavelength by 1 nm. Since the index
change is negative, the wavelength is shortened. (In actuality, the
operating wavelength is also shortened as a result of the increased
threshold current produced by the small amount of optical absorption
introduced by the injected carriers. Since both effects shorten the
operating wavelength, a lower carrier density than calculated above will
actually be required.)
If a wavelength shift much less than 1 nm may be employed, the modulator
region 224 of FIG. 10 may be reversed biased in order to vary the
refractive index via the electro-optic effect. This approach is
advantageous in that the power output of laser 220 will remain nearly
constant between wavelengths without need to adjust the current to the
amplifier region 222. See the aforementioned copending patent application
Ser. No. 07/747,039 for a more detailed discussion of the electro-optic
effect.
In the above discussion it has been assumed that the scanning device (e.g.,
24 of FIG. 1) is a rotating polygon having at least one reflective facet
(e.g., 26 of FIG. 1). However, certain embodiments of the present
invention obviate the need for a scanning device. For example, in the
embodiment 300 shown in FIG. 11, rather than scanning a beam across the
image plane, a line-width beam from source 302 is pixel-by-pixel modulated
by a TIR modulator 304 and projected to an image plane 306 by appropriate
optics 308. The basic configuration of this embodiment is similar to that
shown and described in U.S. Pat. No. 4,638,334 to Burnham et al., dated
Jan. 20, 1987, so that details of the workings of the modulator and
related elements are beyond the scope of this disclosure. However, in
addition to the modulator and related elements, the embodiment of FIG. 11
includes the appropriate apparatus 310, such as a glass prism similar to
that described above, to facilitate line position control in the slow scan
direction on the image plane.
Described above are embodiments employing two distinct methods of
modulation--direct modulation of the light source and modulation by way of
total internal reflection and a zero stop. Other modulation schemes may,
however, be employed without departing from the spirit and scope of the
present invention. For example, another method of modulating beam 16 would
be to project it either onto or through a modulator device (not shown),
such as an electro-optic or acousto-optic modulator, etc. Placement of the
modulator device along the beam path will depend upon its type, the
configuration of apparatus 10, etc., as will be appreciated by one skilled
in the art.
The method of the present invention may utilize either feedback control or
control from stored data, or both, to move the spot in the process scan
direction to accommodate for motion quality errors, and the like. In the
case of feedback control, method and apparatus known in the art would be
employed to determine the actual spot position, the desired spot position,
and any difference therebetween, and to generate from a knowledge of that
difference the proper control signals for effecting the wavelength
adjustment resulting in the desired spot positioning. For example, the
arrangement of FIG. 12 shows a simple method for determining the
rotational error of a photoreceptive drum 12 by way of a synchronized
strobe and sensor arrangement 350 utilizing timing marks 352 on drum 12.
Arrangement 350 includes processing which enables determination of the
existence and extent of rotational error, and generation of a control
signal in response to the determination of the extent of error which is
transmitted to control apparatus and/or current source 354 controlling the
bias applied to the modulator and/or amplifier regions of light source 14.
In the case of control from stored data, the spot position correction is
predetermined. This method is feasible for certain recurrent errors such
as off axis rotation of a photoreceptive drum, surface distortion of a
display screen, etc. The predetermined correction is applied to the
apparatus controlling the bias applied to the light source from a
processor controlled memory device 356 or the like. The output of the
processor controlled memory device 356 would be synchronized by a strobe
and sensor apparatus 350, or other suitable synchronization arrangement,
and may or may not be used in conjunction with a real-time error
determining package such as that described in the preceding paragraph.
FIG. 13 details one complete cycle of operation of the method of the
present invention for correcting for slow scan direction errors. It will
be assumed that any predetermination of required correction for recurrent
errors has been made, and that the correction data has been stored in an
appropriate memory device (not shown). To begin, means (not shown) are
employed to determine whether the current scan line is one for which
predetermined correction data has been stored. This is shown at step 400.
If such data exists, the data is converted into a bias signal which is
applied to the modulator region of the laser or other means for
controlling the laser's output wavelength in order to correct for
predetermined sport position error, as shown at 402. Once the correction
for predetermined errors has been made, or if no such predetermined error
data exists, the light beam is generated at 404. Next, the position that
the beam is incident on the image plane is determined at 406
(alternatively, error in photoreceptor motion or position correctable by
selective spot positioning is determined). If there is slow scan direction
position error at this point, the extent of that error is determined by
appropriate determining apparatus, for example by the aforementioned
strobe and sensor arrangement. The extent of that error is converted to an
appropriate electrical bias signal which is communicated to the modulator
region of the laser or other means for controlling the laser's output
wavelength at 408 in order to correct for the determined error "on the
fly". Once the correction for this has been made, or if it is determined
that no such error exists, the beam may then be scanned and modulated in
order to write the scan line at 410. When the end of scan is detected, a
call is made for the next scan line data at 412, the scan processes in the
slow scan direction and the process begins again at 400.
It will be apparent that, depending on the operating parameters of the
optical output apparatus embodying the present invention, spot position
control may be achieved on a pixel-by-pixel basis. For example, consider a
relatively high performance laser xerographic printing apparatus with
typical operating parameters of 60 page per minute output, 600.times.600
spot per inch resolution, and 14 inch scan. For this device, a typical
pixel exposure time is on the order of 14 nanoseconds. Proper selection of
materials and geometry for the electro-optic element of the present
invention will allows switching speeds of 14 nanoseconds or faster, thus
facilitating mid-line, pixel-by-pixel spot position correction.
By incorporating the above described spot position control methodology with
the appropriate apparatus for xerographic printing, including, for
example, a photoreceptor belt or drum, means for moving the photoreceptor,
means for charging the photoreceptor, means for forming a latent image on
the photoreceptor, means for transferring the latent image to paper, means
for erasing the latent image from the photoreceptor and for cleaning the
photoreceptor, paper transport means, and means for fusing the image onto
the paper, a complete xerographic print engine may be produced. Details of
the structure and operation of printer devices in general are beyond the
scope of the present disclosure, however they are well known to those
skilled in the art. It will be appreciated from the above description,
though, that the present invention is particularly well suited for
inclusion in those printing applications employing ROS as a portion of the
printing process, as well as other printing applications.
In general, to those skilled in the art to which this invention relates,
many changes in construction and widely differing embodiments and
applications of the present invention will suggest themselves without
departing from its spirit and scope. For example, the present invention
operates equally well, and without significant modification, to control
spot position in a single beam ROS or, en bloc, spot positions in a
multiple beam ROS. Furthermore, it is possible to fold the optics of a ROS
apparatus incorporating the present invention, and thereby compact the
apparatus, by employing a reflective or transmissive/reflective body, as
opposed to the aforementioned purely transmissive body, as the dispersive
element. Further still, the apparatus and method of the present invention
may be combined with other apparatus and/or methods of controlling spot
position to achieve advantageous results. For example, the inventions of
copending applications Ser. Nos. 07/747,039 and 07/747,176 may be employed
herewith such that all spots emitted from a multiple laser array are
selectively positioned by those inventions to correct for motion quality
errors, while bow correction may be accomplished on individual spots by
the present invention. In fact, the present disclosure has detailed
several methods of shifting the wavelength of the light source in order to
control spot position, but it will be appreciated that other methods and
apparatus for wavelength control may be employed without departing from
the spirit and scope of the present invention. Thus, the disclosure and
description herein are illustrative, and are not intended to be in any
sense limiting.
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